This invention relates to the field of fiber optics. More particularly, this invention relates to shaped highly birefringent optical fibers and methods for their manufacture.
Optical fibers that maintain a polarized signal in an optical fiber, referred to as polarization maintaining (PM) fibers, are described, for example, in U.S. Pat. No. 4,896,942. Optical fibers that polarize light from a non-polarized or partially polarized light source, referred to as polarizing (PZ) optical fibers, are described, for example, in U.S. Pat. No. 5,656,888. PM and PZ fibers are used in many different applications, such as sensors, inline fiber devices, Raman lasers, and the like. To polarize or maintain a polarized signal in an optical fiber, the light guiding properties of the core of the optical fiber must be highly birefringent. An elliptical core may cause the anisotropic fiber geometry responsible for this high birefringence. However, this anisotropy is more commonly achieved by depositing or locating adjacent the core diametrically opposed sections of cladding material(s) with substantially higher or lower thermal coefficients of expansion than the outer fiber regions. The diametrically opposed regions define one of the highly birefringent fiber""s two transverse orthogonal polarization axes and decouple the components of the wave traveling along the fiber. In a polarizing fiber, one of the decoupled components is leaked to the cladding and completely attenuated, leaving a single linearly polarized wave. In contrast, a polarization maintaining fiber retains both of the orthogonal signal components with virtually no cross-coupling or loss of signal strength.
Typical highly birefringent fiber designs have two perpendicular planes of symmetry. One plane of symmetry passes through the center of the fiber core and its two diametrically opposed cladding regions. The second plane of symmetry, which is normal to the first plane of symmetry, also passes through the center of the fiber core.
Referring to FIGS. 1(a)-(c), a conventional modified chemical vapor deposition (MCVD) process is shown that may be used to make a collapsed optical fiber preform to be drawn into a PM or PZ optical fiber. Referring to FIG. 1(a), a starting preform 10 includes a fused silica support tube 12 with a known refractive index. An optional outer cladding region 14 made of materials with a refractive index either less than or equal to the refractive index of the support tube 12 is deposited on the inside of the tube 12. The outer cladding region 14 is typically a relatively pure deposition region that prevents migration of contaminants from the support tube 12 into the interior regions of the optical fiber. Inside the outer cladding region 14 is a stress region 16 formed by layers of glass with a high thermal coefficient of expansion. The stress region 16 has an index of refraction that approximately matches the index of refraction of the cladding region 14. In longer wavelength PM designs, an optional inner cladding (Iclad) region 18 may be incorporated between the stress region 16 and a core region 20. The inner cladding region 18 has an index of refraction that is closely matched to the index of refraction of the outer cladding 14 in these PM designs. In PZ designs, the inner cladding 18 is normally a narrow depressed index region. The core region 20 has an index of refraction sufficiently higher than the index of refraction of the surrounding regions to ultimately create a waveguiding region 21 needed for single mode operation at the design wavelength. The waveguiding region 21 typically includes the core 20 and the region immediately adjacent the core, but FIG. 1(a) illustrates a more general case in which the waveguiding region 21 includes the core 20 and at least one other region between the core 20 and the support tube 12.
Referring to FIG. 1(b), the starting preform 10 of FIG. 1(a), which has a substantially circular cross-section, is then ground equally on opposite sides 24, 26 to form a ground preform 22 with a non-circular outer periphery, also referred to herein as a non-circular cross-sectional geometry. In this grinding step a substantial amount of the wall thickness of the support tube 12 is removed, and, in some instances, even the outer cladding region 14 may be ground away. The exact amount of material removed in the grind will affect the cutoff wavelength characteristics and the polarizing holding properties of the fiber that is ultimately drawn from the ground preform 22, and as such is a carefully controlled parameter in the fiber manufacturing process.
Referring to FIG. 1(c), the ground preform 22 of FIG. 1(b) is drawn at high temperature (typically, about 21000 C. to about 2200xc2x0 C.), which causes the ground sides of preform 22 to xe2x80x9ccircularizexe2x80x9d into an optical fiber 30 with a substantially circular cross-section. The circularized optical fiber 30 has an outer cladding 34 and a stress region 36, each with a substantially elliptical cross-section, surrounding an inner cladding region 38 and a core region 40, each with a substantially circular cross-section. Normally, the stress region 36 is made of low melting temperature materials that become fluid during the draw process. This allows the relatively soft outer cladding 34 and the fluid stress region 36 to assume an elliptical cross-sectional shape as the outer fiber region made up of the fused quartz support tube 32 circularizes due to surface tension effects. The inner cladding region 38, if present, retains its substantially circular cross-section, as does the core region 40, to provide, along with the elliptical outer cladding and stress regions, a waveguiding region 31.
The waveguiding region of the PM or PZ optical fiber may also have a core region with a non-circular cross section, such as an ellipse or a rectangle. However, a fiber with a non-circular core design is difficult to splice or connect to conventional round core fibers and generally does not develop sufficient birefringence for more demanding applications.
To maintain or preserve the polarization properties of a signal in an optical fiber, the optical properties of the PM or PZ fiber must be anisotropic. The differing cross-sectional profiles of the layers of the waveguiding region formed by the cladding and core regions in the fiber define two transverse orthogonal axes, which permit the de-coupling of waves polarized along those axes. If a signal launched into these fibers has its polarization aligned with one of these transverse axes, the polarization tends to remain aligned with that axis as the signals are propagated though the fiber. This preserves the polarization of the signal.
PM and PZ fibers often require precise alignment of their transverse orthogonal axes when they are joined to other similar fibers or interfaced to other polarized sources or detectors. For example, to join a PZ fiber with a polarized light source having a known polarization orientation, a polarizer is used to launch light into the fiber, and either the fiber or the polarizer is rotated to identify the axes of maximum and minimum light transmission. The axis of maximum transmission is then aligned with the known polarization orientation of the source. The ratio between the maximum light transmission and the minimum light transmission is referred to as the extinction ratio. To join a PM fiber with another PM fiber, a polarized source or a detector, a similar procedure is used, which requires a polarizer at the fiber input and an analyzer at the fiber output. In this process both the analyzer and the polarizer are rotated to locate the maximum and minimum transmitted power. Both of these procedures require time, optical sources, detectors, lenses, translation stages etc. to identify the axes. Lens tracing techniques can also be used in which light is injected through the side of the fiber and the intensity pattern is scanned on the opposite side to identify the asymmetry. Again, this requires many of the same active components.
In one aspect, this invention is a method for making a shaped, highly birefringent optical fiber, which includes providing a preform with a substantially circular cross section. This preform has a waveguiding region with a core and a cladding adjacent the core. The core has a substantially circular cross section and the cladding has a substantially elliptical cross section. The outer surface of the preform is modified to create a shaped preform with a non-circular cross section. An optical fiber is then drawn from the shaped preform at a temperature and draw rate sufficient to provide an optical fiber with the non-circular cross section of the shaped preform.
In another aspect, this invention is a highly birefringent optical fiber including a waveguiding region having a core with a substantially circular cross section, and a shaped outer periphery with a non-circular cross-section. The optical fiber preferably includes a cladding adjacent the core, and the cladding has a substantially elliptical cross section. The optical fiber preferably has an aspect ratio of about 1:1 to about 2:1, and the elliptical cladding has an ellipticity of about 0.20 to about 0.70.
In another aspect, this invention is a method for connecting a highly birefringent optical fiber to a device. The highly birefringent optical fiber includes a waveguiding region having a core with a substantially circular cross section, wherein the optical fiber has a shaped outer periphery and non-circular cross-sectional geometry. The device includes a connection region shaped to accept the outer periphery of the optical fiber, and the optical fiber is engaged with the connection region in the device. The interconnection of the fiber and the device rotationally aligns the fiber with respect to the device, and further alignment steps are not required.
The inventive method makes possible the manufacture of a highly birefringent optical fiber with a waveguiding region having a cladding with a substantially elliptical cross-section and a core with a substantially circular cross-section. In addition, this method provides control over the shape of the outer periphery of the fiber that is independent of the cross sectional geometry of the highly birefringent waveguiding region. The non-circular cross- sectional shape of the optical fiber made by this method, which may include an outer surface with one or more flat sides, bumps, slots and the like, preferably has a known orientation to the transverse axes of the waveguiding region of the fiber. The non-circular cross-sectional shape of the fiber provides an easily visible, xe2x80x9cpassivexe2x80x9d means of locating the fiber""s transverse, orthogonal birefringent axes, which allows the fibers to be easily aligned with other PM or PZ fibers, sources or detectors without time consuming alignment steps and expensive equipment.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.